A study on the effect of Lewis acid catalysis on the molecular mechanism of the cycloaddition between (E)-methyl cinnamate and cyclopentadiene

The molecular mechanism of the Diels–Alder reaction between (E)-methyl cinnamate and cyclopentadiene has been analyzed by means of AM1 semiempirical method and compared to the experimental results. Stationary points for two reactive channels, endo-cis and exo-cis, on potential energy surfaces have been characterized. Three Lewis acids, boron trifluoride (BF3), aluminum trichloride (AlCl3) and catechol boron bromide (CBB) have been used as catalysts taking into account the formation of a complex between the boron or aluminum atom and the carbonyl oxygen of (E)-methyl cinnamate. The molecular mechanism of the uncatalyzed reaction corresponds to a concerted process. In the presence of BF3 and AlCl3, enhancement of both the asynchronicity and charge transfer, with small decreased energy barriers, were obtained. With CBB, the molecular mechanism changes and the reaction takes place along a stepwise mechanism. The initial step corresponds to the nucleophilic attack of cyclopentadiene on the dienophile double bond to give an intermediate, followed by its closure in the second step, yielding the corresponding final cycloadduct. The inclusion of the CBB catalyst drastically decreases the energy barrier associated with the carbon–carbon bond formation of the first step, relative to the concerted process. The role of the Lewis acid catalyst emerges from the analysis of the results.